General networking

IP tables

Check if rule exist

See: https://superuser.com/questions/360094/how-can-i-check-if-an-iptables-rule-already-exists

NAT

See: https://www.karlrupp.net/en/computer/nat_tutorial

Replace source address in incoming message

sudo iptables -t nat -A INPUT -p udp -s 10.0.1.21 -j SNAT --to-source 10.1.23.55